Today, electrophotographic printing systems, such as digital copiers, are widely used in many business and commercial environments. In a digital copier, an image of a document is captured by an input scanning facility, and converted into selected digital image data. This image data is then typically compressed by a control unit, then either forwarded to a suitable printing module or stored in system memory.
In general these image forming systems contain a plurality of solenoids. The solenoids perform many different functions within the image forming system. In order to improve the quality and reliability of manufactured machines, the health of the various components that constitute the machines is important. The most common failure mode of a solenoid is that it does not pull-in all the way or drop-out all the way in a specified amount of time.
One common technique for measuring solenoid health is by computing the time required for the solenoid plunger to pull-in from an initial position of extension, or drop-out from an initial position of retraction. If the solenoid plunger moves in an amount of time equal to a predetermined "pull-in" time, or "drop-out" time, then the solenoid is deemed to be functioning properly. This time measurement technique assumes two things: the solenoid plunger actually began at the full out position, and the solenoid plunger actually ended at the full-in position, or vice-versa.
Experience, however, has shown that an improperly functioning solenoid can appear to be functioning properly if solely judged by the pull-in time. For example, in situations where a solenoid fails to drop-out completely because the frictional force between the plunger and solenoid barrel is too great for the return spring to overcome, the measured pull-in time may still meet specification. This is because the plunger, which is not actually pulling in the full and complete stroke, is moving a less than complete distance in the predetermined amount of time according to specification. The converse may also occur. For example, in situations where a solenoid may not always pull completely in because it cannot overcome an external load and/or internal friction forces, the drop-out may appear to be normal because movement occurred in the predetermined amount of time according to specification, although the solenoid did not drop-out the full and complete stroke.
Experience has also shown that an improperly functioning solenoid can also appear to be operating properly if solely judged by current values in particular regions of a current waveform. To further clarify, when an improperly functioning solenoid moves from a fully extended position to a fully retracted position, or vice versa, in an inordinate, or excessive, amount of time, the measured current values may still appear correct with regard to solenoid position, even though the solenoid is actually malfunctioning.